a person or thing that sleeps.
a heavy horizontal timber for distributing loads.
Building Trades.
any long wooden, metal, or stone piece lying horizontally as a sill or footing.
any of a number of wooden pieces, laid upon the ground or upon masonry or concrete, to which floorboards are nailed.
a sleeping car.
Informal. something or someone that becomes unexpectedly successful or important after a period of being unnoticed, ignored, or considered unpromising or a failure: The play was the sleeper of the season.
merchandise that is not quickly sold because its value is not immediately recognized.
Often, sleepers. one-piece or two-piece pajamas with feet, especially for children.
a sofa, chair, or other piece of furniture that is designed to open up or unfold into a bed; convertible.
Also called sleep, sand. a globule that forms at the inner corner of the eye, especially during sleep, from the accumulated secretion of the glands of the eyelid.
any of several gobioid fishes of the family Eleotridae, of tropical seas, most species of which have the habit of resting quietly on the bottom.
Slang. a spy; mole.
Slang. a juvenile delinquent sentenced to serve more than nine months.
Bowling. a pin that is hidden from view by another pin.
Chiefly British. a timber or beam laid in a railroad track, serving as a foundation or support for the rails; tie.

1175–1225; Middle English; see sleep, -er1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
sleeper (ˈsliːpə)
1.  a person, animal, or thing that sleeps
2.  a railway sleeping car or compartment
3.  (Brit) US and Canadian equivalent: tie one of the blocks supporting the rails on a railway track
4.  a heavy timber beam, esp one that is laid horizontally on the ground
5.  chiefly (Brit) a small plain gold circle worn in a pierced ear lobe to prevent the hole from closing up
6.  a wrestling hold in which a wrestler presses the sides of his opponent's neck, causing him to pass out
7.  (US) an unbranded calf
8.  Also called: sleeper goby any gobioid fish of the family Eleotridae, of brackish or fresh tropical waters, resembling the gobies but lacking a ventral sucker
9.  informal a person or thing that achieves unexpected success after an initial period of obscurity
10.  a spy planted in advance for future use, but not currently active

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

early 13c., "one who sleeps, one who is inclined to sleep much," agent noun of sleep (v.). Meaning "railroad sleeping car" is from 1875. Sense of "something whose importance proves to be greater than expected" first attested 1892, originally in Amer.Eng,. sports jargon, probably
from earlier gambling slang (1856) sense of "unexpected winning card." Meaning "spy, enemy agent, etc. who remains undercover for a long time before attempting his purpose" first attested 1955, originally in ref. to communist agents in the West.
Online Etymology Dictionary, © 2010 Douglas Harper
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Encyclopedia Britannica


any of the marine and freshwater fishes of the family Eleotridae of the suborder Gobioidei (order Perciformes). Sleepers, found in warm and tropical regions, are so named because most species habitually lie quietly on the bottom. They are elongated fishes with two dorsal fins and are distinguished from most other gobies in having their pelvic fins separate, rather than joined to form a weak, rounded suction cup

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
He was that rarity, which you see a lot in fiction but rarely in real life-a
  sleeper agent.
If you waken a sleeper and he tells you his dream, he s reporting from memory.
The sleeper parched of his dreams, or purged of his nightmares, goes swiftly
  bonkers: without fantasy there is no reality.
He went from a sleeper pick for the nomination, to a wise-guy's best bet, to a
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